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Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea
Author(s) -
Soerensen A. L.,
Schartup A. T.,
Skrobonja A.,
Bouchet S.,
Amouroux D.,
LiemNguyen V.,
Björn E.
Publication year - 2018
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2018gb005942
Subject(s) - anoxic waters , methylmercury , environmental chemistry , water column , mercury (programming language) , genetic algorithm , bioaccumulation , chemistry , hypoxia (environmental) , environmental science , oxygen , oceanography , ecology , geology , biology , organic chemistry , computer science , programming language
Oxygen‐depleted areas are spreading in coastal and offshore waters worldwide, but the implication for production and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation experiments to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concentrations were 2–6 and 30–55 times higher in hypoxic and anoxic than in normoxic water, respectively, while only 1–3 and 1–2 times higher for total Hg (THg). We systematically detected divalent inorganic Hg (Hg II ) methylation in anoxic water but rarely in other waters. In anoxic water, high concentrations of dissolved sulfide cause formation of dissolved species of Hg II : HgS 2 H − (aq) and Hg (SH) 2 0 (aq) . This prolongs the lifetime and increases the reservoir of Hg II readily available for methylation, driving the high MeHg concentrations in anoxic zones. In the hypoxic zone and at the hypoxic‐anoxic interface, Hg concentrations, partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concentrations, in these zones. We find that zooplankton in the summertime are exposed to 2–6 times higher MeHg concentrations in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.